Hydrogen is a key chemical used in many petroleum and petrochemical operations. Typically it is used in upgrading and finishing many refinery products. The hydrogen used in these processes sometimes is recovered as a by-product of another refinery process such as alkane reforming to aromatics. Another source of the hydrogen is via the steam reforming of a hydrocarbon such as methane.
In a steam reforming process, steam is reacted with a hydrocarbon containing feed to produce a hydrogen-rich synthesis gas. The general stoichiometry, as illustrated for methane, is:CH4+H2O--->CO+3H2  (1)
Because of the high endothermicity of the reaction, steam reforming is typically carried out in large furnaces, in which the catalyst is packed into tubes. The tubes must withstand the high pressure of the produced synthesis gas, while transmitting heat at temperatures approaching 1000° C. As described in Stanford Research Institute International Report No. 212 (1994), steam reforming process efficiency, defined as the heat of combustion of product hydrogen divided by the heat of combustion of reforming feed and furnace fuel, is approximately 74%, while the space velocity, defined as Standard Cubic Feet per Hour of C1-equivalent feed/ft3 of catalyst bed is 1000 hr−1. Unfortunately, steam reforming furnaces occupy a very large volume of space, orders of magnitude greater than the tube volume, such that low productivity limits the economic attractiveness of the process. Thus, key limitations of the steam reforming process are the relatively low efficiency to hydrogen and the large volumes occupied by the steam reforming furnaces.
Sederquist (U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353) teaches a steam reforming process in which the heat of reforming is provided within the bed by cycling between combustion and reforming stages of a cycle. As described by Sederquist, the high quality of heat recovery within the reforming bed results in a theoretical efficiency of about 97%. However, the examples and commercial projections within these patents describe a process that operates at very low productivity, with space velocities of around 95 hr−1 (as C1-equiv). Moreover, this process requires a compressor to compress the product synthesis gas to useful pressures for hydrocarbon synthesis.
Recently a highly efficient and highly productive process for producing synthesis gas in a cyclic, packed-bed operation has been discovered. In this process, the reforming step involves preheating a first zone to a temperature in the range of about 700° C. to 2000° C. and then introducing a 20° C. to 600° C. hydrocarbon-containing feed, along with steam and optionally CO2 to the inlet of the first zone. Upon introduction of the reactants, the hydrocarbon is reformed into synthesis gas over a catalyst in this first zone. The synthesis gas is then passed from the first zone to a second zone, where the gas is cooled to a temperature close to the inlet temperature of the hydrocarbon feed. The synthesis gas is recovered as it exits the inlet of the second zone.
The regeneration step begins when a gas is introduced to the inlet of the second zone. This gas is heated by the stored heat of the second zone to the high temperature of the zone and carries the heat back into the first zone. Finally, an oxygen-containing gas and fuel are combusted near the interface of the two zones, producing a hot flue gas that travels across the first zone, thus re-heating that zone to a temperature high enough to reform the feed. Once heat regeneration is completed, the cycle is completed and reforming begins again.
An advantage of this process is the ability to operate the reforming step at a higher pressure than the regeneration step, thus creating a pressure swing, and producing high pressure synthesis gas.
In the generation of hydrogen via steam reforming of a hydrocarbon the stoichiometry shown in equation 1 is typically altered by subjecting the product stream to the so called water shift reaction illustrated by equation 2:CO+H2O--->CO2+H2  (2)
The practical application of any hydrogen generation process will depend upon how well the various stages of the process can be integrated into an overall process design. The invention described herein provides a process scheme for generating hydrogen at improved thermal efficiencies and that is particularly adaptable for environments requiring hydrogen at relatively high pressures for refinery processes, for direct use as a fuel and for distribution.